MODERN CONCEPTIONS OF THE UNIVERSE. 63 
One instance will show how the progress of discovery compels 
a wider outlook. Newton accounted for the motions of the 
planets and their satellites by the law of gravitation, and the 
work of subsequent mathematicians and astronomers has 
abundantly verified his formula in the case of bodies of con- 
siderable mass. But recent investigation has verified the 
theoretical prediction of Maxwell that, when one of the bodies 
is intensely heated, as the sun is, the stream of radiant energy 
which falls upon the second body exerts a force upon it. As 
the second body becomes smaller, this force rises in importance 
relative to the force due to gravitation till at leneth it rivals 
and surpasses it, and it follows that the motions of those 
particles of cosmical dust, which are scattered through space, 
depend not only upon the action of gravitation, as was formerly 
supposed, but also upon the pressure of radiation, 
Modern discoveries have led us to a point of view from which 
we are compelled to regard every particle in the universe as 
continually subject to a great variety of actions, though of 
course at any given instant some actions may be more strongly 
in evidence than others, and thus we realize that the history of 
even a single molecule, considered as a whole, is one of great 
complexity. 
The evidence of the spectroscope indicates that each molecule 
has a very complex structure. Thus, each line in the spectrum 
of a substance corresponds to one mode of vibration of the 
molecules, and in the spectra of some substances, such as iron, 
hundreds of these lines may be counted. But the molecules are 
not merely complex in themselves; they have very complex 
connexions with their surroundings. Thus oxygen can combine 
with nearly all the other chemical elements either singly, as in 
the case of hydrogen in the formation of water, or in groups, as 
in the case of hydrogen and sulphur in the formation of 
sulphuric acid. The total number of such combinations is 
enormous. Thus we may say that each element is so 
constructed as to respond to the influence of each of the great 
majority of the other elements, and to a great number of their 
compounds. Of recent years the discovery of radio-active 
substances has greatly raised our estimate of the complexity 
of molecules. 
When we combine the complexity of each molecule with the 
vastness of the number of molecules in the world of stones and 
trees and men and sun and stars, and consider that each mole- 
cule acts on every other one, the complexity of the conception 
is enough to make us despair of further progress. But science 
